In an array transducer ultrasound imaging system, the ultrasound transducer includes an array of active transducer elements. To support this array of transducer elements, the system includes a plurality of parallel channels, wherein each channel includes a transmitter and a receiver connected to one of the active transducer elements in the array. The transducer elements are arranged in a regularly spaced array. Each transmitter outputs an ultrasound pulse through a transducer element into an object to be imaged, typically the human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted by each element in the array so that the transmitted energy arrives at a desired point in phase, thus the energy adds constructively at that point. This causes a portion of the pulse to be reflected back to the transducer array by various structures and tissues in the body.
Steering and focusing of the received ultrasound energy is effected in similar manner. The ultrasound energy reflected from the structures or tissues arrives at the different array elements at different times according to the distance from the structure. The received signals are amplified, delayed and then summed in a receive beamformer. The delay for each element is selected such that the reflected energy received by each transducer from the desired point is input into the summing unit in phase (at the same time), thus creating a received beam that is focused at the desired point. The delays may be varied dynamically so as to focus the beam at progressively increasing depths, or ranges, as the ultrasound energy is received. The transmitted beam can be scanned in a region of the body, and the signals generated by the beamformer are processed to produce an image of the region.
One important consideration in ultrasound imaging is the image sequence rate, or scan rate. A pulse of ultrasonic energy directed from the ultrasonic imaging system to the region of interest has a finite round-trip propagation time. The depth of the region of interest and the propagation velocity through the tissue are factors which determine the round-trip propagation time. For reasons known in the art, a subsequent pulse of ultrasonic energy cannot be transmitted until energy returning from a previous pulse has been received, so the round-trip propagation time sets a limit on the maximum pulse rate. If only one point of interest is isolated per pulse, then the round-trip propagation time also sets a limit on the system's maximum scan rate. The scan rate is particularly important for color Doppler imaging of blood flow and for producing images with higher lateral resolution at a high image rate.
One approach to increasing the scan rate is to receive beams from more than one direction at the same time within the spread of the single transmitted pattern. Another approach is to simultaneously transmit sound patterns along directions widely spaced and to simultaneously receive beams from one or more directions within the spread of each transmitted pattern. In prior art systems, multiple receive beams are formed by multiple beamformers operating in parallel. However, because of the large amount of circuitry required for each beamformer, this approach is expensive and must be used only when each one of the simultaneously received beams fully utilizes the forming means of the beamformer.
FIG. 1 shows a block diagram of a prior art M-channel multi-beam ultrasound front end and beamformer system 100 that utilizes J active transducer elements and is capable of receiving N beams simultaneously. The signals from the active transducer elements are processed through a switching network 110 and applied to appropriate channel for further processing. Each channel includes a signal conditioning unit 120 (designated in the Figure as "Pre-condition"), a digitizing unit 130, and N delay/apodization units 140a-140n ("delay units"). Each channel also includes N two-input summing units (or adders). The switching network 110 receives an ultrasound signal from each of the ultrasound transducers and selectively directs the signals to the signal processing elements. The switching network 110 allows the system to have fewer processing channels than transducers, so that a set of processing channels can sequentially process signals of transducers from multiple regions of the transducer array.
In each channel, the Pre-condition unit 120 receives an analog signal from the switching network 110 and filters or conditions that signal. The output signal generated by the Pre-condition unit is sampled or digitized by the digitizing unit 130, and the resulting digital samples are applied to each of the delay units 140a-140n. In each of the channels two through M, the output signal generated by the (k)th delay unit 140 of the channel is applied to the first input of the (k)th adder 150 of the channel for all k from one to N while the second input of the (k)th adder of the channel is coupled to receive the output signal generated by the (k)th delay unit in the previous adjacent channel. Thus, in the (j)th channel, the second input of the (k)th adder is coupled to receive the output signal generated by the nth adder in the (j-l)th channel, for all channels from two to M, and for all beams from one to N. In the (j)th channel, the (k)th adder generates an output signal representative of the (k)th received beam, for all beams from one to N. All channels utilize an identical architecture; however, since the first channel has no previous channel's output available, the output of the delay units of the first channel are each added to a value equivalent to a zero or null signal level.
The group of M delay units formed by summing the (k)th delay unit from each of the M channels constitutes a "beam former" for controlling the receive angle of the nth beam, so the system uses N beam formers to control the reception of the N beams. The adders in the channels thus form N summing trees. The (k)th summing tree sums the output signals generated by the (k)th beam former in all of the M channels, for all n beams from one to N.
FIGS. 2a and 2b show a block diagram of the system 200, 201 described in U.S. Pat. No. 5,469,851, issued Nov. 28, 1995 to Lipschutz (hereinafter referred to as the '851 patent). This system achieves the same result as the system shown in FIG. 1 but with fewer components. In addition to a pre-condition unit 220 and a digitizing unit 230, each channel includes a single Time Division Multiplexed bus (hereinafter referred to as the TDM bus) and a single TDM Delay and Apodization Unit 240, and rather than including (M.times.N) adders, channels one through M each include a single TDM Summing Unit 250. In each channel, the TDM Delay and Apodization Unit 240 effectively replaces the N delay units 140 included in the FIG. 1 system when the processing means of the N delay units included in the system of FIG. 1 are used for only 1/N of their processing rate. The TDM summing units 250 are synchronized with the TDM delay units 240, so that the TDM summing unit 250 in the Mth channel generates a summed. TDM output signal that is representative of all N received beams. Replacing the N beam formers of (FIG. 1, which are only partially used at any one time) with a single TDM beam former of FIG. 2 (fully utilized) in this fashion decreases the number of hardware components required to implement the system and significantly decreases the cost of the system.
FIG. 2b shows a block diagram of an alternative system 201 having a different architecture than the one shown in FIG. 2a. Instead of including a TDM summing unit in each of channels one through M and summing the channels in series, a single parallel TDM summing unit 250 accepts all M TDM outputs and generates a TDM output signal that is representative of all N received beams. In one embodiment of this prior art device, the system includes 128 channels which are divided into 8 groups of 16 channels. Each group of 16 channels is configured to include 16 TDM Summing Units as shown in FIG. 2a to produce 8 separate TDM beam sums. The 8 TDM beam sums are combined by single TDM parallel summing unit similar to the system shown in FIG. 2b.
TDM systems such as the one exemplified by the '851 patent produce a TDM data stream at a data rate directly proportional to the number of beams being processed. In these systems, low data rates are desirable at the circuit interface between the summing units and the image processing system because the detrimental, frequency dependent effects associated with design and fabrication become more pronounced at higher frequencies. Efforts to mitigate these effects often result in higher production costs.
Accordingly, it is an object of this invention to provide an improved ultrasound. beamformer for processing received signals from an ultrasound transducer array.
It is another object of this invention to provide an improved ultrasound beamformer for processing received signals from an ultrasound transducer array which converts a stream of delayed time multiplexed samples into separate streams of samples corresponding to two or more beams.
It is yet another object of this invention to provide an improved ultrasound beamformer for processing received signals from an ultrasound transducer array which converts a stream of delayed time multiplexed samples into separate streams of samples corresponding to two or more beams in which the data rate of the separate streams is one half the data rate of the time multiplexed samples.
It is a further object of this invention to provide an improved ultrasound beamformer for processing received signals from an ultrasound transducer array which converts a stream of delayed time multiplexed samples into separate streams of samples corresponding to two or more beams in which the data rate of the separate streams is twice the data rate of the received signals.